Referring to FIG. 1, a ducted fan gas turbine engine (e.g. a jet engine) generally indicated at 10 has a principal and rotational axis 11. The engine 10 comprises, in axial flow series, an air intake 12, a propulsive fan 13, an intermediate pressure (IP) compressor 14, a high-pressure (HP) compressor 15, combustion equipment 16, a high pressure turbine 17, an intermediate pressure turbine 18, a low pressure (LP) turbine 19 and a core exhaust nozzle 20. A nacelle 21 generally surrounds the engine 10 and defines the intake 12, a bypass duct 22 and an exhaust nozzle 23.
The gas turbine engine 10 works in the conventional manner so that air entering the intake 11 is accelerated by the fan 13 to produce two air flows: a first airflow A into the intermediate pressure compressor 14 and a second airflow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 14 compresses the airflow A directed into it before delivering that air to the high pressure compressor 15 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 17, 18, 19 before being exhausted through the nozzle 20 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 17, 18, 19 respectively drive the high and intermediate pressure compressors 15, 14 and the fan 13 by suitable interconnecting HP, IP and LP shafts 30, 31, 32.
As shown in FIG. 2, a front bearing housing assembly 40 for the LP shaft 32 on a previously-proposed aero-engine is located forward of the IP compressor 14. The front bearing housing assembly 40 comprises an LP shaft bearing mount system 50, LP phonic wheel 60 and LP shaft axial location bearing 52. The LP phonic wheel 60 provides a signal to the control system to monitor the rotation of the LP shaft 32.
Furthermore, the previously-proposed aero-engine comprises a fan catcher assembly 70 mounted to an LP stub-shaft 32′ (which is rotatably linked to the LP shaft 32) in the front bearing housing assembly 40. The LP phonic wheel 60 is bolted to a fan catcher ring 72, via bolts 62, with the catcher ring 72 being part of the fan catcher assembly 70. The fan catcher ring 72 is in turn connected to the stub shaft 32′ via a nut stack 74.
In the event of a fan-shaft failure or fan blade off event causing damage or failure to the fan-shaft, the fan-catcher assembly 70 is intended to arrest any forward movement of the fan-shaft by impacting on a rearwards end 54 of the LP stub-shaft bearing mount system 50. The resulting reaction load is taken through the LP bearing 50 to the front bearing housing structure 40.
As depicted, the phonic wheel 60 on the previously-proposed aero-engine is part of the fan catcher system 70. If any distortion or damage is caused to the phonic wheel it is important that the phonic wheel remains sufficiently intact that it may continue to provide a slowing down signal for at least approximately five engine revolutions or 200 ms after this event. Failure to provide at least a slowing down signal may mean that the control system ignores the loss of signal, instead assuming that the probe has simply failed. This is because the control system cannot differentiate the loss of signal from a probe failure or from a probe failure caused by a fan-shaft failure or fan blade off event. The control system may therefore continue to supply fuel to the combustion system and the turbine may continue to drive. Consequently, due to the fan shaft breaking and the resulting loss of inertia, there is a risk that the turbine may over-speed.
It is therefore desirable that the phonic wheel 60 remains intact and active to report a signal to the engine controller for a defined period of time after such a failure. Otherwise the controller does not recognise the fan shaft failure event, allowing continuation of fuel feed to the engine, which results in a potential turbine shaft over-speed.
As shown in FIG. 2, the phonic wheel 60 of the previously-proposed arrangement is clamped or bolted to the fan catcher ring assembly 70 (and hence LP stub shaft 32′) by means of bolts and/or a locknut arrangement 62. However, upon impact the loads are sufficient to cause the fan catcher ring 72 to unload the clamping loads of the bolts and locknut 62 such that the fan catcher ring may pivot or bend back into the phonic wheel 60. Because the phonic wheel is not isolated from this distortion, this action may detach the phonic wheel from the catcher ring 72 and therefore prevent the speed signal from being maintained.
Furthermore, the fan catcher ring 72 is not supported in a way that adequately restricts precession or pivoting of the catcher ring relative to the stub shaft 32′. As the axial load is applied to the catcher ring 72 during a fan shaft failure, the clamp load on the catcher ring 72 is reduced and this may allow slippage on the axial face between the stub shaft 32′ and the catcher ring 72. This prevents bending of the catcher ring 72 which helps to restrict the axial deflection of the ring. This high heeling deflection of the catcher ring 72 impacts directly on the phonic wheel mount, which in the previously-proposed arrangement is bolted directly to the catcher ring. These bolts 62 fail under such loading causing the phonic wheel 60 to become detached. However, simply removing the bolts 62 and fastening the phonic wheel 60 to the stub shaft 32′ within the nut stack 74 will not stiffen up the catcher ring sufficiently to reduce the ring deflection, and so the phonic wheel will be destroyed as well. It is therefore desirable to reduce the axial deflection caused by the pivoting or heeling of the catcher ring within the nut stack.
The present disclosure therefore seeks to address these issues.